Research into the oil agglomeration of bituminous coals as a means of beneficiation has been very successful. However, transfer of this technology to low-rank coals has proven difficult. Put another way, low-rank coals, that is coals which contain inherent and particulate ash and are high in moisture content are very difficult to agglomerate. This is important because much of the available coal resources in this country are surface mined lignite and subbituminous (low-rank) coals. These are particularly known to be high in organic salt content and high in mineral content, high moisture, and some are high in sulfur. They also burn less efficiently, and may cause ash fouling of boilers and may cause more undesirable emissions.
Nevertheless, the dwindling petroleum reserves and OPEC control of the petroleum economy in the past two decades has rekindled interest, worldwide, in coal as a source of energy. The return to emphasis on coal utilization after several decades of a petroleum energy base has begun to impact the world's supply of high quality, easily mined, low-ash coal. Mineral matter (including organic salts), and sulfur and water content are three of the major concerns with the increased use of coal as a utility fuel or as a feedstock for conversion processes. Their presence in the coal impacts ash handling and disposal, SO.sub.x and NO.sub.x emissions, fly ash and ash fouling, calorific value of the fuel, transportation costs, and the reactivity of the feedstock. Substantial upgrading of the coal would result from its demineralization and dewatering. Finding new, more efficient methods of beneficiation of coals has therefore become a priority in coal research.
Among the most widely used methods of fine coal beneficiation at present is froth flotation. Although the technique works quite well with higher rank coals and fines with larger particle sizes and low ash, it does have some drawbacks. Among its shortcomings are a comparatively low yield of beneficiated product with a high moisture content when fines are less than 6 .mu.m. As mined, coals and coals with oxidized surfaces are not amenable to beneficiation by flotation.
Agglomeration studies of lignite and subbituminous (low-rank) coals have not met with a great deal of success primarily because the experiments have centered around the successful techniques used to agglomerate bituminous coal. Since agglomeration is a surface phenomenon, the binding oil selected to form the aggregates of fines must be compatible with the surface functional groups on the fines. Subbituminous and lignite coals contain large amounts of surface oxygen organic salts, minerals and water making their surfaces less oleo-philic than the surfaces of bituminous coals.
Most oils used for agglomeration are not highly polar and as a result are readily adsorbed to the organic surface of the coal particles, provided they have minimal surface polar groups and surface water. These characteristics apply to bituminous coals but not to lower rank coals. Since the theory of agglomeration assumes mineral material is considerably more hydrophilic and olephobic than the organic coal matrix, the mineral material will dissolve or form a suspension in an aqueous medium and the organic matter, upon mixing with a limited amount of oil, will form aggregates and separate from that phase. Again, this is more easily accomplished with aliphatic binding oils for the coals of higher rank than for those of lower rank.
It can therefore be seen that there is a continuing and real need for the development of coal beneficiation processes which can be used with low-rank coal. Moreover, there is a continuing need for processes which are cost effective, provide agglomerates which are nonpolluting, and which in fact beneficiate the low-rank coal.
The process of the earlier cross-referenced Knudson and Timpe invention had as its primary objective the fulfillment of the above need. In particular, the development of an economical low-rank coal beneficiation process, which could be successfully performed and which would reduce the polluting effect of the coal, as well as the salt content, mineral content and moisture content, such that it can be more efficiently used. This was accomplished with a particular coal derived oil for agglomeration of low-rank coals. The process of the present invention continues to improve on this earlier process with a series of further process steps to further beneficiate low-rank coals.
The series of additional process steps of the present invention improve the economics of use of low-rank coal, improve the economics of the coal processing, and enhance the low ash, low moisture aspects of the agglomerated product.
The most important objective of the present invention is to continue the development of low-rank coal as an alternative to oil which can be efficiently used without pollution problems.
The method and manner of accomplishing the above objective as well as others will become apparent from the detailed description of the invention which follows hereinafter.